Fix executableDir() return type

Author: aweebit

lib/command.js

@@ -1964,7 +1964,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
    * program.executableDir('subcommands');
    *
    * @param {string} [path]
-   * @return {string|Command}
+   * @return {string|null|Command}
    */
 
   executableDir(path) {

typings/index.d.ts

@@ -819,7 +819,7 @@ export class Command {
   /**
    * Get the executable search directory.
    */
-  executableDir(): string;
+  executableDir(): string | null;
 
   /**
    * Output help information for this command.

typings/index.test-d.ts

@@ -275,7 +275,7 @@ expectType<commander.Command>(program.nameFromFilename(__filename));
 
 // executableDir
 expectType<commander.Command>(program.executableDir(__dirname));
-expectType<string>(program.executableDir());
+expectType<string | null>(program.executableDir());
 
 // outputHelp
 // eslint-disable-next-line @typescript-eslint/no-invalid-void-type, @typescript-eslint/no-confusing-void-expression